York Air Handler: MP, MX, MV, AHP, AHX, AVY

Re: Auxiliary Heaters Not Working

The 4HK heater kits for these air handlers consist of In-line beakers, a control board, snap-disc limits, and DC voltage relays. The control board has two heat stage inputs, "W1" and "W2".

In some instances, "W1" and "W2" are wired together to energize all heater on a call for Aux Heat or Defrost (see wiring diagram for outdoor unit model)

Cases where the customer reports cold air in Defrost, or the system is unable to maintain temperature in cold conditions - it is likely that some (or all) of the heater banks are not working.

• MP, MX, AHP, AHX Air Handlers use Board P#: 031-09156-000
• MV, AHV Air Handlers use Board P#: 031-09157-000

Checkout: (Assuming indoor fan is operating)

1. Check "Heat" Switch on Air Handler Control Board

• The air handler control board has a pin selector to enable or disable electric heat.
• The air handler and replacement control boards come from the factory set for "No Heat".
  • In the "No Heat" position, the heat pump can operate in the heating mode but the electric heaters will not energize.
  • The pin must be set to "Heat" for the electric heaters to operate. 

2. Check Air Handler Breakers

• Set meter to 600 VAC
  • Check for 200 - 230V Outlet Power at each air handler breaker.
    • No or Low power ⇒ Check Breaker Inlet Power
      • No Inlet Power ⇒  Check Breaker Box
      • Inlet Power + No or Low Outlet Power = Bad Air Handler Breaker

3. Check Low Voltage

• Check for 24V power from "W1" to "C" and "W2" to "C".
  • Power Present ⇒ Continue
  • No Power ⇒ Check for 24V power from "R" to "C"
    • Power on "R" + No Power on "W1" or W2" = No Call for Heaters ⇒ Re-check the thermostat
• Check the air handler control board's LED light.
  • No Light = No Power or Bad Board
  • Flashing Light = Fault
    • With both "W1" and "W2" powered and no fault indication, all heaters should be on.

4. Check Heater Amperage

• Use an Amp Probe to check amperage on each heater (approx. 18 amps per heater).
  • Power on "W1" & "W2" + All Heaters Not On → Step 5. Check Board, Relays & Heaters

5. Check Board, Relays & Heaters

• Check the DC Voltage input on each relay
  • The relays are energized by 22VDC from the control board, so be sure to set meter to DC Voltage when testing coil voltage.
    • "Purple" to "Brown
    • "Yellow" to "Brown"
    • "Gray" to "Brown"
      • If it is too difficult (or dangerous) to access the heat relays, an alternative would be to test voltage at the molex plug of the air handler control board.
      • No DC Voltage + No Fault Codes = Bad Control Board
• Check Line Voltage Power through the relays
  • Be sure to set meter to 600VAC.
  • Check from Relay Inlet Power to opposite side of breaker.
  • Check from Relay Outlet Power to opposite side of breaker. 
    • 200-230V In + Not Out = Bad Relay
    • 200-230V In and Out + No Heater Amperage = Bad Heater or Blown Fusible Link ⇒ Check for voltage on each side of the Fusible Link.
      • Power On Both Sides + No Amperage = Bad Heater
      • Power on One Side Only = Blown Fusible Link